Cellulosic fibers are a basic component of absorbent articles such as diapers. These fibers form a liquid absorbent structure, a key element of the absorbent article. Cellulosic fluff pulp, a form of cellulosic fibers, is a suitable fiber for this application because a high void volume, or high bulk, liquid absorbent fiber structure is provided. This structure, however, tends to collapse during wetting, and this reduction in fiber structure bulk reduces the volume of liquid that can be retained in the wetted structure, and also inhibits the wicking of liquid into the unwetted portion of the cellulose fiber structure. Consequently, the potential capacity of the dry high bulk fiber structure is not realized and it is the fiber structure's wet bulk that determines the liquid holding capacity of the overall fiber structure.
Fiber structures formed from crosslinked cellulosic fibers generally have enhanced wet bulk compared to those formed from uncrosslinked fibers. The enhanced bulk is a consequence of the stiffness, twist, and curl imparted to the fibers as a result of crosslinking. Accordingly, crosslinked fibers are advantageously incorporated into absorbent articles to enhance their wet bulk.
Polycarboxylic acids have been used to crosslink cellulosic fibers. For example, absorbent structures containing individualized cellulosic fibers crosslinked with a C2-C9 polycarboxylic acid are described in U.S. Pat. No. 5,137,537, U.S. Pat. No. 5,183,707, and U.S. Pat. No. 5,190,563, among others. Absorbent structures made from these individualized, crosslinked fibers exhibit increased dry and wet resilience, and improved responsiveness to wetting, relative to structures containing uncrosslinked fibers. Furthermore, citric acid, a monomeric polycarboxylic acid, is available in large quantities at relatively low prices making it commercially competitive with formaldehyde and formaldehyde-addition products, or urea-glyoxal condensation products.
However, cellulosic fibers crosslinked with monomeric polycarboxylic acids such as citric acid tend to lose their crosslinks over time and revert to an uncrosslinked state. For example, citric acid crosslinked fibers show a considerable loss of crosslinks on storage. Such a reversion of crosslinking generally defeats the purpose of fiber crosslinking, which is to increase the fiber's bulk and capacity. Thus, the useful shelf-life of fibers crosslinked with these monomeric polycarboxylic acids is relatively short and renders the fibers somewhat limited in their utility.
In contrast, polymeric polycarboxylic acid crosslinked fibers, such as those disclosed, for example, in U.S. Pat. No. 6,620,865, exhibit a density that remains substantially unchanged over the lifetime of fibrous webs prepared from these fibers. This resistance to aging or reversion of density relates to the stable intrafiber crosslinks formed using such polymeric crosslinking agents. Cellulose fibers crosslinked with citric acid show a considerable increase in density, accompanied by a loss of bulk and absorbent capacity over time. Generally, the increase in density indicates a decrease in the level of crosslinking (i.e. reversion) in the fibers. In addition to density increase, the loss of crosslinking in the fibrous web results in a less bulky web and, consequently, diminished absorbent capacity and liquid acquisition capability.
Some crosslinking agents can cause discoloration, i.e. yellowing, of the white cellulosic fibers at the elevated temperatures required to effect the crosslinking reaction. A possible mechanism, at least for citric acid, is a dehydration reaction resulting in aconitic acid and a yellow coloration attributable to the C═C chromophore.
Widespread consumer demand for brighter, whiter pulp drives manufacturers to pursue methods for reducing discoloration. Bleaching, for example, is a common method for increasing pulp brightness (as defined by the Technical Association of the Pulp & Paper Industry (“TAPPI”) or the International Organization for Standardization (“ISO”)). Industrial practice for improving appearance of fluff pulp is to bleach the pulp to increase its brightness. Traditional bleaching agents include elemental chlorine, chlorine dioxide, and hypochlorites. However, bleaching, especially with chlorine-containing agents, can be environmentally harsh, expensive, and a source of manufacturing bottleneck. Accordingly, there have been many attempts to reduce the number, nature, and quantity of bleaching agents used in bleaching methods.
Also, while highly bleached pulps are “whiter” than less-bleached material, such pulps are often still yellow-white in color. Consumer studies indicate a clear preference for blue-white over yellow-white colors, as the former is perceived to be whiter, i.e. fresh, new, and clean, in comparison to the latter, which is considered to be old, faded, or dirty.
Addressing this preference, U.S. Pat. No. 7,513,973, for example, suggests that whiteness attribute, rather than TAPPI or ISO brightness, better correlates with consumer preference for product whiteness, and discloses bleaching methods that seek to improve the Whiteness Index of cellulosic fibers crosslinked with polymeric polycarboxylic crosslinking agents, such as methods that involve the use of sodium hydroxide and/or hydrogen peroxide.
It is generally accepted that the active mechanism in chromophore elimination in bleaching operations that include hydrogen peroxide involves the perhydroxyl ion OOH−. The formation of the perhydroxyl anion can be enhanced, for example, by increasing the pH during the bleaching stage, according to the following reaction:H2O2+OH−⇄OOH−+H2O
Accordingly, industry practice is to perform hydrogen peroxide bleaching in alkaline systems, such as by adding an alkaline agent (such as sodium hydroxide) in coordination with hydrogen peroxide in the bleaching stage.
In U.S. Pat. No. 5,562,740, the combination of an alkaline agent (such as sodium hydroxide) with an oxidizing agent (such as hydrogen peroxide) has been observed, in a bleaching stage at a pH of at least 5.5, to reduce the “smoky and burnt” odor of cellulosic fibers crosslinked with alpha-hydroxy carboxylic acid crosslinking agents such as citric acid, in addition to improving brightness. The '740 patent theorizes that the malodor is due to the collective presence of a host of substances including volatile phenolics, hydrogen sulfide, various sugar decomposition products (such as furfural, methyl furfural, and guaicols), and citric acid anhydrides, and that the odor reduction may be due to the possible reduction of such substances by the combination of alkaline and oxidizing agents at a pH of at least 5.5.
Although this “burnt” odor originally became identified as a characteristic of citric acid crosslinked cellulosic fibers specifically, such as in the '740 patent, it has become known that the “burnt” odor also accompanies cellulosic fibers crosslinked with polymeric polycarboxylic acids, such as polyacrylic acid, as well.
Accordingly there remains a need for absorbent articles comprising cellulose fibers crosslinked with polymeric polycarboxylic acids, which cellulose fibers exhibit high bulk, increased brightness and whiteness, and reduced malodor.